1. Field of the Invention
The present invention relates to a power steering system for automotive vehicles, particularly of the kind having a control valve which is adapted to shut off supply of working oil or fluid to a power cylinder when a steering wheel is in the neutral position, i.e., when steering in the straight-ahead position.
2. Description of the Related Art
This kind of power steering system as for example disclosed in PCT pamphlet WO95/11153 is known.
The above described prior art power steering system is constructed such that working fluid is supplied to or discharged from a power cylinder in response to a steering effort on an input haft of a steering gear for thereby providing a steering assist. The power steering system is thus provided with a control valve for controlling supply of working fluid to the power cylinder and shutting off the supply of working fluid to the power cylinder when steering in the straight-ahead position. A pair of such control valves are provided so as to be operated selectively in response to a steering effort for turning to the right and to the left, respectively.
The above described prior art power steering system is constructed so that one of the control valves opens in response to a steering effort for turning to the left or to the right, e.g., in response to a steering effort for turning to the left to introduce working fluid to a left-hand cylinder chamber of a power cylinder and thereby provide a steering assist for assisting the steering effort for turning to the left, whereas the other of the control valves opens in response to a steering effort for turning to the right to introduce working fluid to a right-hand cylinder chamber of the power cylinder and thereby provide a steering assist for assisting the steering effort for turning to the right.
However, since the above described prior art power steering system requires two control valves, one of which opens at the time of steering for turning to the left and the other opens at the time of steering for turning to the right, it is liable to become larger in the overall size, requires a number of constituent parts and thus possibly increases in manufacturing cost.